The present invention relates to electric equipment requiring supply of backup electric power during an OFF state of a power supply switch, and power supply circuit structures for use in such electric equipment.
In recent years, numerous pieces of electric equipment containing a microcomputer and semiconductor memory have been in practical use. In electric equipment of this type, like DSP amplifiers, the semiconductor memory, such as a SRAM (Static Random Access Memory), provided within or separately from the microcomputer normally requires supply of backup electric power to retain at least part of its storage function even after the power supply switch of the equipment is turned OFF.
Typical examples of the conventionally-known backup power supply scheme include the following.
(a) One where, as shown in FIG. 7, when a power supply switch SW connected with the primary winding of a transformer T is turned OFF, electric charge stored in a capacitor or condenser C connected with the secondary winding (or a separate secondary cell) is used as a backup power supply for a microcomputer and the like. PA1 (b) One where, as shown in FIG. 8, a sub-transformer T2 is provided, separately from a transformer T of the main power supply, to comprise an auxiliary or backup power supply. Here, the primary winding of the sub-transformer is connected to a supply plug at a point closer than a power supply switch SW. Thus, even after the power supply switch SW is turned OFF, the backup power supply remains active as long as the plug is maintained in connection to the electric outlet. PA1 (c) A compromise between the above-mentioned two schemes, where the sub-transformer provides a power backup while the supply plug is in connection to the electric outlet, and the condenser is caused to provide a power backup only when the supply plug is disconnected from the electric outlet.
However, the above-mentioned three conventional backup power supply schemes are not satisfactory for the following reasons. First, the scheme mentioned in item (a) above presents the problem that if the power supply switch remains turned OFF for a long time period, the condenser etc. would discharge electricity to thereby invite an unwanted loss of stored data although no substantial electricity is consumed by the backup power supply. In addition, deterioration of the condenser etc., occurring with the passage of time, would lead to a reduced backup time. Therefore, the other two schemes employing the sub-transformer, mentioned in items (b) and (c) above, are more popular nowadays.
But, with the item (b) and (c) schemes, there is encountered the problem that substantial electricity consumption by the sub-transformer is unavoidable, even though the power backup can be retained as long as the plug is kept in connection with the electric outlet. While the necessary power consumption for retaining stored data in the microcomputer etc. is nominal, usually on the order of .mu.A at most, the exciting current has to be constantly supplied to the sub-transformer even during an OFF state of the power supply switch, which would result in a power loss as great as several hundred mW that can never be ignored from the viewpoint of energy conservation.